1. Field of the Invention
The present invention relates to ink jet printing systems and, more particularly, to a drop-on-demand type ink jet printhead having a page-wide array of piezoelectric actuators.
2. Description of Related Art
Ink jet printing systems use the ejection of tiny droplets of ink to produce an image. The devices used in ink jet printing systems produce highly reproducible and controllable droplets, so that a droplet may be printed at a location specified by digitally stored image data. Most commercially available ink jet printing systems may be generally classified as either a "continuous jet" type or a "drop-on-demand" type. In the "continuous jet" system, ink droplets are continuously ejected from the printhead and either directed to or away from the paper depending on the desired image to be produced. In the "drop-on-demand" type ink jet printing system, ink droplets are ejected from the printhead in response to a specific command related to the image to be produced.
In the drop-on-demand type ink jet printing systems, transient pressures in the fluid are induced by the application of a voltage pulse to a piezoelectric material which is directly or indirectly coupled to the fluid. These transient pressures cause pressure/velocity transients to occur within the fluid and these pressure/velocity transients are directed to produce a droplet that issues from an orifice. Recently, considerable interest has been directed to piezoelectric drop-on-demand type ink jet printheads which utilize sidewall actuators to impart droplet ejecting pressure pulses into the ink carrying channels. See, for example, U.S. Pat. No. 4,536,097 to Nilsson, U.S. Pat. No. 4,879,568 to Bartky et al., U.S. Pat. No. 4,887,100 to Michaelis et al., U.S. Pat. No. 5,016,028 to Temple, U.S. Pat. No. 5,227,813 to Pies et al. and U.S. Pat. No. 5,235,352 to Pies et al. The Bartky et al., Michaelis et al. and Temple patents further disclose shear mode sidewall actuators characterized by extending the poling direction normal to the widthwise direction of the page. Both of the patents to Pies et al. disclose shear mode sidewall actuators characterized by extending the poling direction in the widthwise direction of the page.
The printhead configurations disclosed in the Pies et al. patents may be manufactured in accordance with the techniques disclosed in U.S. Pat. No. 5,433,809 to Pies. In accordance with this technique, the side surfaces of an unpolled thin piece of piezoelectric material are electroded and a voltage applied there across to pole the thin piece. Once polled, these electrodes are stripped off and a layer of conductive material is deposited on the top and bottom side surfaces of the thin piece to enable shear mode excitation. The thin piece of piezoelectric material is conductively mounted to a base and a series of sidewalls. The series of sidewalls are produced by forming parallel grooves which extend through the thin piece and part of the base piece; for example, by using a sawing process.
One drawback to such a method of manufacture is that the technique is only suitable for manufacturing an ink jet printhead having a relatively narrow widthwise dimension. This method of manufacture cannot be readily applied to the manufacture of page-wide arrays. Specifically, the aforementioned thin piece was poled in the widthwise direction, i.e. the direction generally parallel to the width of the page.
Typically, to properly pole piezoelectric material, a voltage differential on the order of 30 to 75 volts per mil (i.e., per one-thousandth of an inch) is required. Accordingly, to pole a one inch wide piece requires a voltage differential somewhere in the range of 30,000 and 75,000 volts. This poling voltage requirement currently limits the manufacturable width of an ink jet printhead body to about two inches since an appreciably wider piezoelectric body section would require an unacceptably higher poling voltage. For example, an eight and one-half inch (or "page") wide piezoelectric printhead would require a poling voltage somewhere in the range of 255,000 to 637,500 volts. Even if this much wider PZT body section could be properly poled at this extremely high voltage, the material would tend to crack during or upon completion of the poling process for the PZT body section.
This PZT printhead body width limitation has resulted in the inability to manufacture piezoelectric ink jet printheads in full page, i.e. eight and one-half inch, widths. This necessitates the shuttling back and forth of a relatively small width, i.e., one inch, piezoelectric printhead across a print medium sheet interiorly traversing the ink jet printer. While acceptable for many uses, such small width or "shuttle-type" ink jet printheads are generally characterized by numerous disadvantages that render them less desirable than page-wide devices. One disadvantage of shuttle printheads is that printing speed is restricted since two mechanical steps, printhead movement across print medium and print medium progression, are required. Further, because ink must be selectively ejected from the shuttle printhead as the print medium is progressing through the printer and the printhead is simultaneously moving across the paper medium, the print quality obtainable with such a printhead may be affected due to difficulties of timing the ejection of ink in coordination with the movement of the print medium and the mechanical movement of the printhead.
An additional disadvantage of the shuttle printhead is that such a printhead cannot easily be utilized where multiple colors or types of ink are to be used. This difficulty occurs because the shuttle printhead is typically fed by a single ink source or reservoir. To feed a shuttle printhead with more than a single color or type of ink, print speed or quality must be sacrificed. In particular, if multiple ink colors or types are simultaneously utilized in the narrow width printhead, the total number of ejection nozzles of the printhead must be allocated among the multiple colors or types of ink. Such an allocation reduces the number of nozzles available for printing with any one of the colors or types of ink at a given time. An alternative to the allocation of ejection nozzles for multiple colors or types of ink would be a mechanism employed for switching ink sources to the printhead from time to time during the printing process so that only a single ink of the multiple colors or types of ink available feeds the printhead at any given time. In each of these cases, however, the limitations of speed or quality are encountered when seeking to employ the multiple colors or types of ink. One solution to overcoming these limitations might appear to be widening of the printhead to allow multiple ink colors or types to be employed simultaneously or otherwise; however, as previously discussed, widening the printhead has typically resulted in a host of other problems.
As a result, it has been further contemplated that several such printheads be physically attached to each other to form a page-wide device. U.S. Pat. No. 5,365,645 to Walker discloses a method by which plural two-inch wide blocks of piezoelectric material are stitched together to form a single page-wide array. The difficulties associated with stitching several blocks of piezoelectric material into a single page-wide array, however, adds considerable cost to the manufacture of such a device. Furthermore, such techniques raise some concerns as to the uniformity of channels that extend across the boundary between two pieces of stitched piezoelectric material. Finally, positional inaccuracies resulting from misalignments during the stitching of multiple narrow printheads also occur.
It would thus be an improvement in the art to provide a page-wide printhead that could simultaneously deliver multiple ink colors or types without loss of speed or print quality.
Thus, it is desired to provide a drop-on-demand type ink jet printhead having a page-wide array of piezoelectric actuators. Accordingly, it is an object of the present invention to provide such a printhead and an associated method of manufacturing the same.